The present invention relates to a process for treating the surface of aluminum foil for use as an electrode of electrolytic capacitors, and more particularly to an improved process for etching aluminum foil to enlarge the surface of the foil to the greatest possible extent and obtain an increased capacitance.
It is desired that the aluminum foil useful as an electrode of electrolytic capacitors have the largest possible surface area and a great capacitance per unit volume. Generally, the aluminum foil for this use has heretofore been etched electrochemically or chemically and thereby increased in its substantial surface area. Various improvements have been made in etching processes for forming a larger number of enlarged etching pits of increased depth in order to achieve a maximum area increase ratio.
A group of inventors including one of the present inventors has already proposed an improved etching process in Japanese Patent Application No. 55-143214 (Published Unexamined Japanese Patent Application No. 57-66615). The proposed etching process of the prior art comprises two divided steps of electrolytic etching. In the pre-etching step, aluminum foil is electrolytically treated in a solution of hydrochloric acid and an acid, such as sulfuric acid or chromic acid, which electrolytically forms a porous oxide film on the surface of the foil, to form a large number of relatively deep etching pits in the surface while inhibiting overall dissolving of the foil surface. The aluminum foil is then subjected to electrolytic post-etching treatment in an aqueous solution of hydrochloric acid and an oxidizing acid, whereby the etching pits formed by the pre-etching step are concentrically attacked and thereby enlarged and deepened. While this process is very effective for achieving a high area increase ratio, we carried out experiments and research to find that when the pre-etching treatment is conducted in an aqueous solution of hydrochloric acid, and chromic acid or a chromate, a larger number of etching pits of increased depth can be formed most effectively with the foil surface inhibited from dissolving. However, we found that the process then involves a problem. When the pre-etching treatment is conducted using the aqueous solution which contains chromic acid or chromate, the chromate film formed on the surface of the aluminum foil acts to inhibit etching, making it impossible for the post-etching step to enlarge the etching pits in diameter to a fully satisfactory extent.